Superconducting magnets and inductors are capable of generating high magnetic fields and thereby storing large amounts of energy. Superconducting inductors are very efficient for these purposes because no energy is lost to resistive or joule heating in the superconducting current path. Every superconducting material has a critical temperature T.sub.c for a given ambient magnetic field above which the material is no longer superconducting. If a region of a superconducting material loses its superconducting property (i.e. becomes normal or quenches), joule heating occurs in the normal or nonsuperconducting region. If the region is small enough, the heat will be dissipated and the region will return to its superconducting state.
If the region is large, such that sufficient joule heating occurs and overcomes the system's ability to dissipate the heat, the normal zone will propagate and grow larger, causing a catastrophic condition which can result in severe damage to the inductor or magnet as even more energy is dissipated in portions of the inductor or magnet. This runaway condition can result in the uncontrolled dumping of the entire magnetic energy of the inductor or magnet causing damage to itself and possibly to the load. It could also result in a service outage of the inductor or magnet which could be intolerable if the inductor or magnet where part of a device having a critical military application. Early detection of a quench, however, permits the energy stored in the magnet or inductor to be dissipated in a controlled fashion. The energy can be dissipated in a variety of ways such as through dump resistors or by making the entire magnet or inductor go normal in a controlled fashion. Catastrophic damage due to overheating is therefore avoided.
Generally, a superconducting inductor is any current path composed of a superconducting material since any current path has a self-inductance. Superconducting inductors, and especially superconducting energy storage inductors, are generally configured as coils called solenoids or toroids. Superconducting magnets, which are a special case of superconducting inductors, often have more complex shapes, so as to appropriately shape the magnetic field they produce.
Several techniques exist for detecting and locating normal regions in a superconducting magnet or inductor. The principle technique involves the use of a series of voltage taps. Voltages are measured by voltmeters at various points along the coil of the superconducting material, with the objective of correlating changes in voltage caused by the change in resistivity due to the creation of a normal region. A severe drawback with using voltage taps is that in addition to the resistive voltage associated with a normal zone, a superconducting inductor produces inductive voltages resulting from the charging and discharging of the coil. These "common mode" inductive voltages are variable and change with any changes in the magnetic field. Also, since normal zones must be detected when they are small, the resistive voltage resulting from a quench is very small, typically 10-20 volts; whereas the common mode inductive voltage between two voltage taps is typically much larger and can be tens of kilovolts. If voltage taps are used, some technique must be utilized to eliminate the inductive voltage from the voltage measured at the taps. Typically, this involves subtracting out the inductive voltage by comparing the signal to a reference voltage. In any event, the technique involves subtracting two voltage measurements, one entirely inductive and the other mostly inductive but also having a small resistive component, with the hope of recovering the small resistive component.
As mentioned above, the inductive voltage across the inductor's terminals may be tens of kilovolts during normal operation. This means the sensor must be floated at these high voltages. This places a severe constraint upon any electronic components attached to the voltage taps to measure and detect a quench since these components must be designed to operate at these high inductive voltages. Also, since the components must be placed close to the voltage taps, they must be capable of operating properly at cryogenic temperatures and in high magnetic fields.
Often, numerous voltage taps are used to locate a normal zone. Typically, the taps are placed between turns of the inductor coil. Resolution, however, is limited to locating the normal zone between two voltage taps. Thus, the relative position of the normal zone between the two voltage taps cannot be determined. It would be desirable to be able to pinpoint within a turn where the normal zone occurs, especially since a turn of a coil in a superconducting inductor or magnet for large energy storage can have a diameter of between 600 and 1000 feet.
Another technique for the detector of quenches has been proposed for cable-in-conduit conductors. See L. Dresner, "Quench Detection By Fluid Dynamic Means In Cable-In-Conduit Superconductors" published in 1988 by Plenum Publishing Corp. of New York, N.Y., in the Proceedings of the 1987 Cryogenic Engineering Conference, Vol. 33, at pages 167-174 thereof. A cable-in-conduit conductor is one in which the superconducting conductor is sheathed in a conduit through which a coolant flows. In contrast, a pool cooled conductor sits in a pool of coolant. When a normal zone occurs in a cable-in-conduit conductor, the coolant in contact with the normal zone is heated. As the coolant is heated, a pressure pulse is transmitted along the conduit and is detected at the coolant source or sink of the conduit. This technique has limitations in that it only applies to superconducting inductors which utilize cable-in-conduit conductors There are also difficulties in measuring the pressure pulse since the conduit is a dynamic fluid system. Pressure changes resulting from conditions independent of the creation of a normal zone, such as source pressure, must be taken into account. Moreover, the resolution of this technique is limited in that the relative position of the normal zone along the superconductinq inductor cannot be determined.
It would be desirable, therefore, to develop a sensor for quench detection and location in superconductors under operating and quiescent conditions wherein not only can a quench easily be detected without encountering the difficulties mentioned above, but the exact location of the quench can also be determined.